In an oil immersed electrical apparatus such as an oil immersed transformer, a coil that serves as a current carrying medium is arranged in an insulating oil. This coil may be a copper coil formed of copper. Insulating paper is wound around the copper coil so that the copper coil would not be short-circuited between adjacent turns.
Meanwhile, the mineral oil (the aforementioned insulating oil) adopted for the oil immersed transformer contains sulfur elements. The sulfur elements react with easily-sulfurized metals such as copper and silver that are used in the oil immersed transformer to produce conductive sulfides. For example, a sulfur element reacts with the copper components provided in the oil to precipitate conductive copper sulfide on the surface of the insulating paper. Then, a conductive path is formed between adjacent turns of the copper coil, which may cause dielectric breakdown (see, for example, Non-patent Document 1).
Apart from the precipitation of copper sulfide on the insulating paper, a phenomenon has been known in which copper sulfide precipitates on the surface of metal. When this happens, as the amount of copper sulfide produced increases, the copper sulfide that peels off from the surface of the metal and floats in the insulating oil may lower the insulating performance of the apparatus. As a method of avoiding such a phenomenon, a detection member may be provided in the apparatus to detect production of copper sulfide on the metal surface (see, for example, Patent Document 1). In this method, a detection member is prepared by spraying copper powder onto the surfaces of an epoxy resin insulating plate to be dispersed and fixed, the formation of copper sulfide is detected when the surface resistance of this detection member decreases, and thus an abnormality in the apparatus can be diagnosed.    Patent Document 1: Japanese Patent Application Laid-open No. 04-176108    Non-patent Document 1: CIGRE TF A2.31, “Copper Sulphide in Transformer Insulation”, ELECTRA, No. 224, pp. 20-23, 2006